Microfabrication techniques have long been known to those skilled in the art for production of microelectronic circuits. More recently, such microfabrication techniques have been applied in the production of three dimensional mechanical microdevices for applications, for example, in the fields of micro-optics, mechanical and medical engineering and the like. Using such techniques, microstructures can be fabricated comprising a variety of materials in meeting conductivity and other functional requirements. Microdevices are possible, for example, comprising nickel or nickel-cobalt components for high mechanical stability. Biocompatible materials can be incorporated for biomedical devices. Ceramics also can be incorporated. Opportunities in automotive applications have been identified for miniature motors. Miniaturization offers improved packaging and cost reduction opportunities.
Technologies for microfabrication and the associated miniature devices often are referred to as MEMS for microelectromechanical systems. MEMS technologies include those that employ the thin-film deposition and patterning procedures of the silicon electronics industry. Cost efficiencies can be achieved through batch processing of silicon wafers. MEMS techniques have been used, for example, in bulk micromachining of pressure sensors and accelerometers. More recently, thin films of polycrystallize silicon supported on a sacrificial layer have been used to produce even smaller versions of such devices. Miniature electrostatic motors and electrically driven actuators also have been fabricated using MEMS. A new microfabrication technology known as LIGA (a German acronym taken from words referring to lithography, electroplating and injection molding) expands MEMS to designs based on plastics, metals, alloys and ceramics. LIGA also supports batch processing and uses all of the thin-film technologies of the silicon-based electronics industry. A common denominator of LIGA and silicon micromachining is that both processes typically begin with a photolithography step. In LIGA a two-dimensional pattern is projected into a thick polymer film forming a latent image. This image is then developed by chemical removal of the exposed regions leaving a three-dimensional structure having patterned features with relatively high aspect ratio: wall height divided by feature width. Surface micromachining of poly-Si typically produces features with minimum widths of 1 to 2 microns (.mu.m) and equivalent heights; giving aspect ratios of unity. In the LIGA process minimum feature sizes of 2 .mu.m width by 300 .mu.m height are typical; thus obtaining aspect ratios on the order of 100.
The LIGA process has been commercialized by MicroParts, GmbH (Karlsruhe, Germany), through an association with the Karlsruhe Nuclear Research Center, and has been used to make microdevices with movable parts, for example, microturbines, movable spring elements and acceleration sensors with stationary electrodes and movable seismic mass members. Other exemplary devices produced using such technologies include: a microturbine with a 150 .mu.m diameter rotor; micro-electrostatic motor with 0.6 mm rotor diameter; an electrostatic linear actuator; optic fiber multiplexer/demultiplexer with 10 fibers; copper coils with 20 .mu.m by 100 .mu.m conductors wound with 20 .mu.m spacings; 12-tooth gears 80 .mu.m in diameter by 140 .mu.m height with 28 .mu.m center bore, pulleys, pulley-belts, assembled gear trains and clamps.
Other MEMS technologies applicable to the present invention include those referred to as MPP-MEMS ("micropatterned polymers for MEMS"). Instead of using PMMA resist and synchrotron exposures as in LIGA, the MPP-MEMS approach is to use a photosensitive polyimide as the resist and expose it with ultra violet light. Electroforming is then used to form devises and features in the patterned polyimide. The minimum feature size typically is not as small as in LIGA and the aspect ratio not as high.
The importance of microfabrication is increasing along with general miniaturization trends and a growing need to integrate mechanical, electronic and optical components into microsystems. The present invention provides a planar micro-motor which can be fabricated using known microfabrication techniques, such as those mentioned above, and can be employed as a component of a larger assembly, even being integrated with microcircuitry and other system components.